At least 23 people have been killed and 153 arrested in four days of clashes as Brazilian security forces crack down on drug gangs in Rio de Janeiro’s violent slums.

Heavily-armed men continued to stop cars and buses, rob passengers and set vehicles on fire, police said.

Military police said 15 suspected gang members died in shootouts with officers on Thursday.

Since the weekend, at least 23 people have died in clashes and two policemen have been wounded.

Gang-related violence has plagued Rio for decades, but is mostly centered on the city’s slums, known as favelas. But now it’s spilling onto the wealthier parts of town as drug traffickers are fighting back against police operations aimed at pacifying the slums.

“This is a desperate attempt to weaken our security efforts,” said Sergio Cabral, governor of the state of Rio de Janeiro.

“What they want is to create panic, for society to retreat, but we will not retreat.”

The escalating violence casts more doubt on the ability of Rio authorities to get a grip on security ahead of its hosting of two of the planet’s largest sporting events, the 2014 World Cup and the Olympics two years later.

“There are groups of criminals who have been installed here for 20, 30 years, and they might not want to give up,” said Rio state public safety director Jose Beltrame.

“But we’re not giving up either. If they keep this up, we will redouble our efforts. Anyone who gets in our path will be run over.”

The police have been heavily reinforced by the Brazilian government as more than 1,000 officers were removed from desk jobs to join the operations and 300 extra motorcycle police were on patrol making up a total of 17,500 officers to fight drug gangs.